They 'can' have a wandering zero - but that's when they're hot, because the receiver apparently will warp a bit during Rapid Fire.
Mostly, they're pretty accurate, and the Brits thought about making them their standard rifle until the FAL came to be.
Someone makes a new rubber buttpad, now - and believe me, most of them need the originals to be replaced, because they're hard as rocks.
They also fake the living hell out of them, so if someone wants one, it behooves them to do their homework.